1. Field of the Invention
The present invention provides a method of employing data obtained through magnetic resonance imaging in creating three-dimensional brain fiber reconstructions and, more specifically, it provides such a method which is particularly useful in respect of white matter fibers.
2. Description of the Prior Art
It has been known for many purposes to attempt to image portions of a human or animal""s brain for diagnostic, research, or therapeutic purposes. In order to understand the activity of and relationships between multiple cortical regions of the brain, it is important to analyze their physical connectivities, by axonal fibers. The projections of these fibers can be traced in experimental animals by observing axonal degeneration following carefully placed experimental brain lesions or, alternatively, by injecting and subsequently localizing radio-isotopes or other chemicals which are taken up by nerve cells and actively transported along their axons. Comparable human data are necessarily much more limited because they have been obtained only after postmortem examinations of patients with naturally occurring lesions such as injuries or infarcts.
As a result of the foregoing, progress in understanding the structure of cognitive association pathways and their dysfunction in many disorders has been slow due to the lack of a non-invasive method for fiber tracking. Conventional MR imaging can distinguish gray and white matter, but provides no information about the fiber orientation in white matter. Diffusion-weighted MRI allows in vivo mapping of the anisotropic and isotropic diffusional properties of brain water, and has revealed a high degree of diffusional anisotropy in white matter. Although this finding has been tentatively attributed to preferential water diffusion along axons and/or their myelin sheaths, it has not been known to show tracking of neuronal projections.
There remains, therefore, a need for improved methods of imaging white matter fibers in the brain including the brain of a living human being.
The present invention involves a method of creating an image of brain fibers which includes the data acquisition by the magnetic resonance imaging process and the data processing to generate imaging information relating to the fibers.
The data acquisition by the magnetic resonance imaging contains the acquisition of so-called diffusion-weighted images that are later used for the calculation of apparent diffusion constant at each picture element (pixel) along more than six axes. This can be accomplished by using a pair of magnetic field gradients to sensitize the magnetic resonance imaging. Conventional magnetic resonance imaging scanners are equipped with three magnetic field gradient units. By combining these units and by changing the strength, a series of images sensitized to water diffusion along desired direction can be recorded.
After the data acquisition, the imaging information is transferred to the computer for the fiber analysis. First, intensity of each pixel of diffusion-weighted images with various gradient combination and strength are fitted to calculate six independent variables in a 3xc3x973 diffusion tensor. The diffusion tensor is then diagonalized to obtain three eigenvalues and three eigenvectors. These six values are subjected to the further computer processing to generate images representing the properties of the fibers. This processing preferably consists of three parts: initiation of brain fiber tracking; pixel connecting; and the judgement of the termination of the fibers. For example, a tracking of projections of fibers can be initiated from a point in a three-dimensional space arbitrarily chosen by a user and propagated in both directions according to the direction of the fiber (the eigenvector associated with the largest eigenvalues). Each time the tracking leaves a pixel to the next pixel, judgement is made whether the fiber is continuous or terminated based on randomness of the fiber orientation of the adjacent pixels.
It is an object of the present invention to provide a method for using data acquired by magnetic resonance imaging of a brain in creating a three-dimensional fiber structure for white matter fibers of the brain.
It is a further object of the present invention to provide such a method wherein axonal fiber may be imaged so as to provide images containing details regarding the patient white matter fibers.
It is a further object of the present invention to provide an automated means of converting magnetic resonance imaging data into brain fiber three-dimensional image.
It is a further object of the present invention to provide such a method which may be practiced on living human beings.
These and other objects of the invention will be more fully understood from the following detailed descriptions of the preferred embodiments on reference to the illustrations appended hereto.